We have recently discovered that myosin crossbridges bind to actin in relaxed muscle and interact in a manner characterized by rapid, reversible attachment and detachment. We have also found that the rate of crossbridge attachment and detachment depends upon the nucleotide or nucleotide analogue bound to the active site. Since the attachment/detachment rates in the presence of ATP are so rapid as to make measurement difficult, we have recently concentrated on studying the behavior of actin-myosin crossbridges in the presence of pyrophosphate (PPi) or adenyl-5-yl imidodiphosphate (AMPPNP) where attachment/detachment rates are slower. We detect crossbridge detachment by measuring the decay of tension following rapid stretch of an isolated single muscle fiber which has had its outer membrane removed to facilitate equilibration with experimental solutions. We have found that detachment of the crossbridges in 4, 1, or 0.25 mM PPi or AMPPNP does not proceed with a single rate-constant. Rather, it proceeds over a wide range of rate-constants with the ratio of the fastest and slowest rate-constants as great as one hundred-one thousand. Since the number of crossbridges attached at the end of the force decay is the same as at the start, the simplest interpretation of the force decay is that strained cross-bridges are detaching from one actin and reattaching to another actin in a position of lesser strain. The reason this process proceeds over such a wide range of rate-constants is under intense investigation; it may be related to competition between adjacent myosins for available actin sites. If so, force maintenance in a muscle fiber may be highly cooperative when large numbers of crossbridges are attached. This may have an important role in the regulation of skeletal muscle.